Skeletal Muscle: Structure & Function Lecture Slides (PDF)

Skeletal Muscle: Structure and Function John Warren Learning Objectives Describe Muscle Structure Identify difference in muscle characteristics of fast twitch and slow twitch fibres Define the sliding filament mechanism Explain the process of the cross-bridge cycle Introduction to...

Skeletal Muscle: Structure and Function John Warren Learning Objectives Describe Muscle Structure Identify difference in muscle characteristics of fast twitch and slow twitch fibres Define the sliding filament mechanism Explain the process of the cross-bridge cycle Introduction to skeletal muscle Organ 600 muscles in the human body Comprising 40 – 60 % of total body weight What are the main functions of skeletal muscle? Producing Maintaining Storing and Generating movement posture moving heat substances Muscle Anatomy - Gross muscle anatomy Force produced by the muscle Muscle Force is transmitted to the skeleton via the tendon Movement occurs Tendon or Joint is stabilised or Posture is maintained Muscle Muscle Fibre Sarcomere Structure Each myofibril is composed of thousands of sarcomeres. Joined in series and parallel to one another. Sarcomeres are the functional units of a muscle. Made up of myofilaments… Actin (thin filament) Actin subunits in double helical strands Tropomyosin interacts with actin and covers binding sites where thick filament can bind Important… Myosin (thick filament) Around 300 myosin molecules per thick filament Two subunits: S1 – Globular head S2 – Flexible region and tail Overall structure Structural proteins to be aware of: Titin, Nebulin and Desmin  Take a breathe! Try to write a short paragraph explaining what sarcomere / myosin / actin is in lay terms Post your responses to the Padlet on Learn. I will provide some generic feedback at the end of the week. Sliding filament mechanism Huxley 1954 During contraction filaments slide past each other Each of the two filaments remain relatively unchanged in length despite changes in gross muscle length Sarcoplasmic reticulum Interconnecting tubules surrounding myofibrils Regulates intracellular levels of calcium Stores calcium and releases on stimulation to allow contraction The Cross-bridge Cycle The Cross-bridge Cycle Myosin head picks up another AP arrives ATP and bond with actin is released Repeats until Ca++ or ATP levels drop Pi released from myosin head Ca++ released changes angle of from SR the myosin head POWERSTROKE Hydrolysis of ATP Tropomyosin changes angle of Ca attaches to moves the myosin head. troponin uncovering CROSS-BRIDGE binding sites FORMED  Length – tension relationship A) Filaments overlapped, not able to generate tension. B – C) Muscle stretches, no longer butting against Z line able to produce tension (to an optimal point) D) If the fibre is lengthened further, less overlap between myosin and actin means less opportunity to develop tension Force – velocity relationship Force during shortening < isometric force: The faster the movement the less time myosin heads have to attach to binding site Force during lengthening > isometric force: Compliant portion of myosin stretched further than during isometric force Forcible detachment of myosin heads with stretch Fibre type characteristics Fibre type characteristics Type IIA Type IIB Type I Re – Cap Vevox meeting code 182-709-912 Provide 3 differences in characteristic between type I and type II muscle fibres What are the structural proteins that make up a sarcomere? What is the role of calcium in the crossbridge cycle? Describe a way in which you could prove the existence of the length-tension relationship.  Skeletal muscle II: Innervation and Control John Warren Objectives Name the basic structures of a motor neuron Explain how nerve impulses are transmitted from the spine to the muscle to cause contraction Describe how feedback from sensory receptors can regulate muscle contraction The Nervous System Nervous System Central Nervous Peripheral System Nervous System Afferent Efferent (motor) (Sensory) Somatic Autonomic Sympathetic Parasympathetic Nervous System Nervous System The Motor Neuron Motor neuron – nerve cell Motor unit – a single motor neuron and all of the fibres it innervates Gross movements 2 – 3000 fibres per motor unit Fine movements 2 or 3 fibres per motor units The Motor Neuron Myelin sheath Covers the axon & helps A speed up nerve impulses Cell body The cell’s B life support center Axon C Passes messages (nerve impulses) along The Motor Neuron Node of Ranvier A gap in the myelin sheath of a nerve where the E axon’s membrane is exposed F Terminal branches of axon Form junctions with other cells D Dendrites Receive messages from other cells Transmission of Nerve Impulses Membrane potential (mV) A B C D +30 0 -55 Rest -70 -90 Stimulus Time A single Node of Ranvier Sodium Channel Potassium Channel Myelin Sheath Node of Ranvier 29 At rest At rest more Sodium (Na+) outside than inside the axon 30 When a Nerve Impulse arrives Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ ACTION POTENTIAL arrives Sodium (Na+) channels open up. Na+ enters the axon. 31 Inside of axon becomes more positive Increased Sodium (Na+) within the axon causes the exposed part of the axon between the myelin sheaths to DEPOLARISE & Na+ gates to close. 32 Depolarisation Membrane potential (mV) A B +30 0 A= Sodium gates open A-B= inside of axon -55 becomes more positive -70 B= Sodium gates close -90 Stimulus Time 33 Inside of axon becomes less positive K+ K+ K+ K+ K+ K+ K+ K+ K+ When membrane potential reaches +30 mV: Potassium (K+) gates open. K+ leaves the axon and the inside of the axon becomes less positive. REPOLARISATION phase! 34 Repolarisation Membrane potential (mV) B C +30 0 B= Potassium gates open -55 B-C= Inside of axon becomes less -70 positive -90 Stimulus C= Potassium gates close Time 35 Return to rest Membrane potential (mV) C D +30 0 -55 -70 -90 Stimulus Time C= Potassium (K+) gates close C-D= Sodium (Na+)-K+ pump gates re-establish resting membrane potential 36 Propagation of Action Potential  Neuromuscular Junction Where motor neuron meets the muscle fiber. Motor end plate: pocket formed around motor neuron by sarcolemma. Synaptic cleft: short gap between neuron and motor end plate. Acetylcholine is released from the motor neuron resulting in depolarization of motor end plate (muscle fiber). In summary: Action potential arrives Axon terminal depolarised Calcium influx into axon terminal Encourages release of acetylcholine into the synaptic cleft Acetylcholine interacts with the sodium ion channels on the post synaptic membrane Sodium ion channels open and sodium influx causes depolarisation As the charge of the muscle cell membrane increases, voltage gated calcium channels open and there is a calcium influx The increase in charge also stimulates calcium release from the sarcoplasmic reticulum When the action potential ceases the acetylcholine re-enters the neuron as choline and acetic acid Skeletal Muscle Control Spine Efferent Neurons Definition: Neurons that send impulses from the central nervous system to your limbs and organs (e.g. the muscles) Afferent Neurons Muscle Definition: Neurons that carry nerve impulses from sensory receptors or sense organs toward the central nervous system  Input from brain Golgi Tendon Organs Efferent neuron Senses tension in the tendon when the muscle Afferent contracts neuron Has an inhibitive (negative) afferent neuron When excessively large forces are generated Golgi tendon organ feedback from GTO causes activation of muscle to (GTO) decrease- PROTECTIVE Muscle Spindles Highly specialized encapsulated muscle fibres Input from brain (intrafusal) positioned parallel to normal muscle fibres (extrafusal) Sensitive to changes in muscle length Afferent neuron wraps around the muscle spindle Efferent neuron Efferent neuron (Gamma) cause muscle spindles to contract to maintain tension in middle of the fibres Hence if muscle is stretching rapidly, a vigorous Muscle contraction is caused to prevent overstretching spindle Study questions Explain how nerve impulses are propagated down the axon towards the neuromuscular junction. How does excitation of a neuron result in excitation of a muscle cell? Describe the process with reference to the appropriate neurotransmitter. Describe how feedback from sensory receptors can regulate muscle contraction